CNS disorders are major public health issues. Parkinson's disease (PD) alone affects over 1 million people in the United States. Clinically, PD is characterized by a decrease in spontaneous movements, gait difficulty, postural instability, rigidity and tremor. Parkinson's disease is caused by the degeneration of the pigmented neurons in the substantia nigra of the brain, resulting in decreased dopamine availability. Altered dopamine metabolism has also been implicated in schizophrenic patients who show increased dopamine in certain areas of the brain.
Currently, many CNS disorders such as PD are treated by systemic administration of a therapeutic agent. Systemic administration, however, is often inefficient because of a drug's inability to pass through the blood brain barrier and because many drugs cause peripheral side effects. Thus, many potentially useful compounds, such as proteins, cannot be administered systemically. If these compounds are successful in penetrating the blood-brain-barrier, they may also induce central nervous system side effects as well. Treatment of PD currently involves oral administration of the dopamine-precursor, L-dopa often in combination with a compound such ascarbidopa, a peripheral inhibitor of the enzyme aromatic amino acid decarboxylase (AADC) that decarboxylates dopa to dopamine. In the majority of patients, however, production of AADC in the affected brain regions is reduced as PD progresses and, consequently, larger and larger doses of L-dopa are required, leaving the patients with reduced therapeutic benefits and increased side effects.
In view of the limitations of current systemic therapies, gene delivery is a promising method for the treatment for CNS disorders such as PD. A number of viral based systems for gene transfer purposes have been described, such as retroviral systems which are currently the most widely used viral vector systems for this purpose. For descriptions of various retroviral systems, see, e.g., U.S. Pat. No. 5,219,740; Miller and Rosman (1989) BioTechniques 7:980-990; Miller, A. D. (1990) Human Gene Therapy 1:5-14; Scarpa et al. (1991) Virology 180:849-852; Burns et al. (1993) Proc. Natl. Acad. Sci. USA 90:8033-8037; and Boris-Lawrie and Temin (1993) Cur. Opin. Genet. Develop. 3:102-109.
Adeno-associated virus (AAV) systems are emerging as the leading candidates for use in gene therapy. AAV is a helper-dependent DNA parvovirus which belongs to the genus Dependovirus. AAV requires infection with an unrelated helper virus, either adenovirus, a herpesvirus or vaccinia, in order for a productive infection to occur. The helper virus supplies accessory functions that are necessary for most steps in AAV replication. For a review of AAV, see, e.g., Berns and Bohenzky (1987) Advances in Virus Research (Academic Press, Inc.) 32:243-307.
AAV infects a broad range of tissue, and has not elicited the cytotoxic effects and adverse immune reactions in animal models that have been seen with other viral vectors. (see, e.g., Muzyczka, (1992) Current Topics in Microbiol. and Immunol. 158:97-129; Flotte et al. (1993) PNAS USA 90:10613-10617; Kass-eiser et al. (1992) Gene Therapy 1:395-402; Yange et al. PNAS USA 91:4407-4411; Conrad et al. (1996) Gene Therapy 3:658-668; Yang et al. (1996) Gene Therapy 3:137-144; Brynes et al. (1996) J. Neurosci. 16:3045-3055). Because it can transduce nondividing tissue, AAV may be well adapted for delivering genes to the central nervous system (CNS). U.S. Pat. Number 5,677,158 described methods of making AAV vectors. AAV vectors containing therapeutic genes under the control of the cytomegalovirus (CMV) promoter have been shown to transduce mammalian brain and to have functional effects in models of disease.
AAV vectors carrying transgenes have been described, for example, in Kaplitt et al. (1994) Nature Genetics 8:148-153; WO 95/28493 published Oct. 26, 1995; WO 95/34670, published Dec. 21, 1995; During et al., (1998) Gene Therapy 5:820-827; Mandel et al. (1998) J. Neurosci 18:4271-4284; Szczypka et al. (1999) Neuron 22:167-178.). However, delivery of AAV vectors to the CNS has proven difficult. AAV has been used to transfer the thymidine (tk) kinase gene to experimental gliomas in mice, and the ability of AAV-tk to render these brain tumors sensitive to the cytocidal effects of ganciclovir has been demonstrated. Okada et al. (1996) Gene Therapy 3:959-964; Mizuno et al. (1998) Jpn. J Cancer Res. 89:76-80. Infusion of an AAV-CMV vector containing the human tyrosine hydroxylase (TH) gene, an enzyme involved in conversion of the amino acid tyrosine to dopa, into adult rat brain resulted in transduction of both neurons and glia (Kaplitt et al. (1995) VIRAL VECTORS, GENE THERAPY AND NEUROSCIENCE APPLICATIONS, Kaplitt and Loewy eds., 12:193-210, Academic Press, San Diego; Bankiewicz et al. (1997) Exper. Neurol. 144:147-156). Delivery of the same vector to monkey striatum resulted in robust expression of TH for up to 2.5 months (During et al., supra). Furthermore, AAV-CMV-TH was tested in a rodent model of Parkinson's Disease where it caused significant improvement in rotational behavior of 6-hydroxydopamine-lesioned rats (Fan et al. (1998) Human Gene Therapy 9:2527-2537; Mandel et al. (1997) PNAS USA 94:14083-14088).
However, while reports such as these demonstrate AAV's potential for targeting the CNS, they also demonstrate that direct injection of AAV vectors into the CNS results in limited numbers of transfected cells and that the transfected cells are clustered in a narrow area near the injection tracts. (see, e.g., During et al, supra; Fan et al., supra). Since multiple injections into the CNS cause undesirable complications, there remains a need for methods of delivering AAV vectors to larger areas of the brain using the least number of injection sites. In addition, the relationship between dose of injected vector and its resulting distribution in brain tissue has not been previously reported.
Furthermore, gene therapy of PD has focused on delivery of at least two genes encoding enzymes involved in dopamine synthesis, namely TH and AADC. These methods are subject to all of the delivery problems discussed above and, in addition, require that both genes are expressed in the proper amounts. Thus, treatment of PD using AAV-AADC in combination with L-dopa has also not been demonstrated.